1. Field of the Invention
This invention relates to air-activatable polymerisable compositions and their use. The compositions are suitable for use as one-component systems for forming polymers, requiring no primers. The compositions are useful as adhesives, sealants, surface coatings, moulding resins and composite matrices, for example.
2. Description of Related Art
The cure of acrylic adhesives is initiated by a two-part redox system. The first part, the initiator, is normally present in the base component of the adhesive and the second part is present in the accelerator or curative component. However two-part systems suffer from the disadvantages and limitations of use associated with such systems. It is desirable to provide one-part systems.
Free radical polymerizable compositions are disclosed in European Patent Specification No. 0,356,875 of Henkel KGA. These comprise ethylenically unsaturated, polymerizable compounds together with an activator system which can be initiated by oxygen and water, comprising an N-alkyl-substituted tert-arylamine with at least one aliphatic CH bond in the alpha position, a metal compound generally used for accelerating the drying of unsaturated oil and which is at least partially soluble in the composition, and a compound of a weakly acidic carboxylic acid having a pKa value of no less than about 0.9, which can be hydrolyzed to a free carboxylic acid on contact with moisture.
The N-alkyl-substituted tert.-arylamines correspond in particular to the general formula: ##STR2## in which R.sub.1, is an optionally substituted aryl radical, more especially an optionally alkyl-substituted phenyl radical, R.sub.2, has the same meaning as R.sub.1, or is an optionally substituted, linear or branched alkyl radical and R.sub.3, is a linear or branched alkyl radical which may be substituted, but contains at least one hydrogen atom in the alpha-position to the nitrogen. These compositions depend on the action of both water and oxygen to effect a cure reaction. As such their formulations are very dependant on humidity conditions.
WO91/10687-A of Henkel KGaA discloses air-activatable adhesive compositions comprising mixtures of free-radically polymerisable unsaturated olefinic compounds, and hydrazone compounds prepared and stored under anaerobic conditions. The hydrazones may be alkyl, cycloalkyl and/or aryl hydrazone(s) of aidehyde(s) and/or ketone(s) and are claimed to react with air to form hydroperoxides useful as polymerisation initiators. The one component compositions are stable in the absence of air.
EP 0,502,733A of Loctite (Ireland) Limited (the contents of which are incorporated herein by reference) describes a one-part air-activatable polymerisable composition comprising:
(a) at least one free-radically polymerisable monomer, and PA1 (b) an activator system for effective polymerisation of the free-radically polymerisable monomer, said activator system comprising at least one auto-oxidisable compound of a formula which includes the structure I: ##STR3## where x is 0 or 1, and where the lines representing unfilled valencies indicate bonds to carbon, hydrogen or hetero atoms, alone or in combination with a weak acid; with the proviso that: PA1 a) a free-radically polymerisable monomer, and; PA1 b) an auto oxidisable compound which is a dihydropyridine; PA1 c) An onium salt, and PA1 d) a soluble ionic salt. PA1 (a) a free-radically polymerisable monomer, and PA1 (b) an auto-oxidisable compound which is a dihydropyridine having a general formula selected from II: ##STR4## where R.sub.1 to R7, which may be the same or different, are independently selected from hydrogen, hydrocarbyl and heterohydrocarbyl groups, silyl groups, and substituted derivatives of any of the foregoing, and any two of the groups R.sub.1 to R.sub.7 may together form a mono- or polycyclic ring structure, which may optionally be a fused ring structure, and which in turn may be substituted, provided that none of the groups R.sub.1 to R.sub.7 is a group which is known to interfere with polymerisation, and that in the case of 1,4-dihydropyridines of formula II (iv) the groups R.sub.2 and R.sub.5 at the 3- and 5-positions are not both electron-withdrawing groups, PA1 (c) a tetrazole having the formula III ##STR5## wherein R.sup.8 is selected from hydrogen and hydrocarbyl groups, and (d) a soluble ionic salt, PA1 (i) straight chain or branched linear or alicyclic aliphatic groups including alkyl, alkenyl and alkynyl, preferably containing from 1 to 20, more preferably from 1 to 10, most preferably from 1 to 5 carbon atoms; and alkylene and alkenylene groups forming part of a ring structure which preferably contains from 3 to 30, more preferably 5 to 20 carbon atoms; PA1 (ii) aromatic groups including aryl, alkaryl and aralkyl groups, preferably containing 6 to 20 carbon atoms, more preferably 6 to 10 carbon atoms, and optionally having a fused ring structure.
when x=0, the nitrogen atom in structure I is not bonded to a second nitrogen atom; PA2 and when x=1 there is a structure I in the compound in which the C.dbd.C moiety does not form part of a phenyl ring;
with the proviso that the composition does not contain a peroxide, or a peroxide precursor which produces peroxide in the absence of air or any ingredient which is a significant source of radicals in the absence of air.
Preferred auto-oxidisable compounds are selected from the group consisting of partially hydrogenated pyridines, condensation products of cyclic ketones and ureas, Schiff's bases, indoles, pyrroles, imidazoles, piperazines, carbazoles, tetrahydroquinolines, and substituted derivatives thereof, particularly dihydropyridines.
The most preferred auto-oxidisable compounds are 1,2 -dihydropyridines.
The presence of a weak acid is generally preferred in order to achieve a satisfactory rate of auto-oxidation for the partially hydrogenated pyridines. The majority of the working examples using dihydropyridines in EP 92 301 899.8 have acrylic acid in the formulation. Other acids used are methacrylic acid, trichloroacetic acid, cyanoacetic acid, salicylic acid, benzoic acid, acetic acid and saccharin.
The composition may also contain a soluble ionic salt, particularly a cobalt or iron salt.
The compositions as described in EP 92 301 899.8 are stable against polymerisation during storage in the absence of air, they are activated by exposure to air, and they will then cure in either the presence or absence of air. However the use of volatile acids, particularly acrylic acid, may give rise to odour problems in a working environment.
EP 94301131.1 of Loctite (Ireland) Limited describes an air-activatable polymerisable composition comprising:
While the compositions containing onium salts have value because they do not require the presence of a weak acid such as acrylic acid, there is a need for other solutions to improve the compositions of EP-A-O 502 733 by avoiding the use of volatile acids.
It is well known that alpha, beta- unsaturated carbonyl compounds undergo an addition reaction with conjugated dienes, known as the Diels-Alder reaction. 1,2-dihydropyridines behave as a conjugated diene in a Diels-Alder reaction with acrylic acid. Acrylic acid has a strong electron--withdrawing group close to the vinyl group. There is a risk therefore that compositions which contain a 1,2 -dihydropyridine and acrylic acid may lose activity during storage due to consumption of the 1,2-dihydropyridine in forming the Diels-Alder adduct. Methacrylic acid is not believed to be as susceptible to the Diels-Alder reaction as acrylic acid. However methacrylic acid is not as effective as acrylic acid in activating the dihydropyridines. Further, both acrylic and methacrylic acid are volatile and may give rise to odour problems.